A rare nucleoside that reacts with the N-hydroxysuccinimide ester of phenoxyacetic acid (HSP) will be characterized. This nucleoside is present in E. coli tRNA super Phe, tRNA super Ile, tRNA super Lys, tRNA super Val over 2, tRNA super Arg, and tRNA super Metm. A similar if not identifical nucleoside is present in rat liver tRNA super Tyr, tRNA super Thr, tRNA super Ile, tRNA super Asn and tRNA cuper Cys. The relationship between tRNA modification and growth rate of bacterial cells will be measured. The quantity of this modified nucleoside will be measured by its reactivity with C14 HSP. The quantity of this reactive nucleoside in a particular species of tRNA will be measured by the characteristic change in mobility of the tRNA on BD-cellulose chromatography after the nucleoside has reacted with HSP. The occurrence of this nucleoside in the tRNA extracted from different mammalian tissues will be studied. Factors which affect its rate of synthesis, such as drug and hormone treatments, will be determined. The relationship between the extent of tRNA modification and growth rate will be measured in normal and neoplastic cells. A study of the effect of acylation of this nucleoside on the structure and function of tRNA will be undertaken.